Research On Effect Of Pilot-stage Key Factor On Ignition And Lean Blowout Performance Of Stratified Partially Of Premixed Combustor With High Temperature Rise

The improvement of thermodynamic performance are required for higher thrust-to-weight ratio of future military aircraft engines.Specifically,the combustor,which is usually regarded as the key component,demands higher temperature rise.High temperature rise corresponds to high fuel-air ratio,which means that high temperature combustor is expected to operate over significantly wider fuel-air ratio ranges.Hence,it is the primary factor to balance visible smoke emission at high power conditions and lean blowout fuel-air ratios at low power conditions in the development of high temperature rise combustor.Taking example by the low emission combustor design concept,stratified partially premixed combustion technology show its superiority in solving this problem.The Pilot-stage operate in diffusion combustion mode and the main stage work in partially premixed combustion mode,which is not only able to improve the performance of lean blowout but also avoid visible smoke emission.However,the pilot stage is often neglected in comparison with main stage which accounts for a large proportion of fuel and airflow.So it is necessary to carry out the research on pilot stage systematically and summarize the effect the key factor of pilot stage have on ignition and lean blowout performance as well as combustor performance at high power conditions.Based on the existing swirler scheme of stratified partially premixed high temperature combustor.The study of inner-swirl-angle of pilot stage,outer-swirl-angle of pilot stage,air allocation proportion between inner and outer of pilot stage,divergence angle of pilot stage exit were conducted experimentally and numerically.Primarily,on the basis of the Complex Swirl number(CSn)put forward by author,the effect of different factors on fuel spray distribution and flow fields were studied by means of particle image velocimetry(PIV)and planar laser induced fluorescence(Fuel-PLIF)under atmospheric pressure and temperature.Additionally,Combined with the fuel spray distribution and flow fields,the relationship between various factors and the ignition and lean blow out performances were investigated experimentally.Finally,the influence different elements have on the evaluation criterion of combustor performances such as total pressure loss,combustion efficiency,outlet temperature distribution factor(OTDF)and radial temperature distribution factor(RTDF)were researched numerically.The main conclusion were presented as follows.The PIV and Fuel-PLIF experiment indicate that the inner-swirl-angle of pilot stage will not affect the flow fields within a certain scope.The high speed axially jet-flow occurs when the outer-swirl-angle decreases below a certain threshold,which would break the structure of central recirculation zone(CRZ).The spray cone angle increases when outer-swirl-angle of pilot stage increases,which lead to the increasement of the fuel concentration close to flame tube.The rotation direction between inner and outer of pilot stage almost have no influence on the flow fields.However,in terms of the fuel spray distribution,co-rotating design shows a hollow structure with low fuel concentration in the centre of cone area while the counter-rotating design shows a solid structure with relative high fuel concentration in the centre of cone area.The recirculation zone size and the spray cone angle would decrease when reducing the inner-swirl-angle of pilot stage.In addition,air allocation proportion of inner pilot stage increases within a certain scope will not influence the flow fields.But the CRZ would move downstream when the air allocation proportion of inner pilot stage increases to a certain extent,which lead to the disappearance of recirculation at the exit of the pilot stage.As for the fuel spray distribution,fuel concentration and the axially size of the centre of cone area will expand when increasing air allocation proportion of inner pilot stage.The decline of the divergence angle of pilot stage exit directly result in the existence of the step recirculation zone(SRZ)but have no significant effect on the flow fields.Additionally,spray cone angle also increases when increasing divergence angle of pilot stage exit.The experiment related to ignition and lean blowout shows that ignition capability improves when outer-swirl-angle of pilot stage increases.The effect of increasement of outer-swirl-angle of pilot stage on lean blow out can be almost neglected within a certain scope.However,greatly decrease of outer-swirl-angle of pilot stage would deteriorate the performance of lean blowout remarkably,especially for the low pressure drop of flame tube.In terms of the capacity of ignition and lean blowout,counter-rotating design is better than co-rotating design.Because in comparison with co-rotating configuration the shear effect of airflow between inner and outer pilot stage is relatively strong for counter-rotating configuration,which is beneficial for fuel atomization.In addition,the increase in inner-swirl-angle of pilot stage would lead to the reduction of the recirculation size,which is adversely influencing formation and propagation of kernel flame as well as flame stability and finally deteriorates the performance of ignition and lean blowout.The ignition and lean blowout performance improves when the air allocation proportion of inner pilot stage increases under the condition of low pressure drop of flame tube.However,an opposite trend is found in the case of relatively high pressure drop.The divergence angle of pilot stage exit have no impact on the capacity of ignition and lean blowout.But the decrease of the divergence angle of pilot stage exit contributes to the formation of the SRZ,which is able to avoid carbon deposition on step surface.The numerical simulation results shows that the quality of outlet temperature distribution is improved by decreasing the air allocation proportion of inner pilot stage.But it leads to the increase of total pressure loss.In terms of the combustion efficiency,the configuration with 2/8 air-ratio allocation between inner and outer pilot stage performs best.And the combustion efficiency of the other two schemes are relatively low.Increasing outer-swirl-angle of pilot stage helps to improve the combustion efficiency and outlet temperature distribution.As for total pressure loss,the case that inner-swirl-angle of pilot stage is 25°show the top performance among the three configurations.The co-rotating design and the increasement of inner-swirl-angle of pilot stage are both beneficial for reducing the total pressure loss.But it would lead to the deterioration of outlet temperature distribution,especially for the OTDF.The impact of the inner-swirl-angle of pilot stage is not significant in terms of combustion efficiency and the RTDF.Increasing the divergence angle of pilot stage exit helps to reduce total pressure loss and improve combustion efficiency,as well as reducing the OTDF and RTDF and improving the quality of outlet temperature distribution effectively.